The present invention relates to a system for the control of tonal noise emissions in wind turbines. The present invention further relates to a wind energy system including the same.
Wind energy systems have gained more and more importance as a source of energy in recent years. As areas suitable for their economical operation are relatively scarce in a number of countries, a growing number of wind energy systems are located close to populated areas. As the running of wind turbines in wind energy systems causes noise, this leads to various problems, such as resistance from neighborhoods and the like. A variety of factors contribute to the overall noise emission of such systems, of which one is the tonal emission caused by moving mechanical parts. These parts may for example be the generator or other parts of the drive train which tend to resonate at resonant frequencies. A well-known source for tonal emissions are gearboxes, which is mainly due to the occurrence of teeth-meshing frequencies. The tonal emission behaviour of mechanical parts in the drive train depends on a variety of factors, of which some are hardly influenceable by a manufacturer. Often, the only possibility to improve tonal emission characteristics of a wind turbine is to exchange the noise generating parts, e.g. a gearbox or a generator. However, this causes high costs for the owner due to downtime, exchange costs and costs for the new equipment. Thus, what is needed are cost efficient measures to reduce the tonal emissions caused by a wind energy system.
Methods for reducing the noise emissions of wind energy systems have long been discussed. For example, it was proposed to reduce aerodynamic noise caused by the blades through a speed/torque control of the system in order to keep turbine speed low during certain time intervals, e.g. during night time. Others have proposed to reduce speed/torque of the wind turbines in a wind park individually in order to gain maximum overall performance of the system while avoiding that a part of the wind turbines runs at significantly higher speeds than the average speed of all turbines. Another example for the application of a speed/torque control, however not related to noise reduction, is the proposal to quickly steer a wind turbine through ranges of rotor speed which typically cause low frequency resonances of the wind turbine tower to avoid structural damage.